The invention relates generally to wireless communication systems and, more particularly, to methods for performing handoffs in wireless communication systems.
A satellite communication system is a system that provides communication services to subscribers using communication satellites orbiting about the earth. The satellites act as nodes in a large scale communication network, each node being able to direct communication traffic flow through the network in accordance with a predetermined routing plan. In a typical communication connection in a satellite communication system, a first user communicates with a remote user via a series of satellite up-links, cross-links, and down-links. That is, a first two-way wireless link is established between the first user and a first satellite in the constellation, one or more satellite cross-links are used to provide communication between the first satellite and a remote satellite, and a second two-way wireless link is established between the remote satellite and the remote user. The communication signals (e.g., data packets) flowing between the first user and the second user will generally include routing information that specifies the particular route though the system that the signal is to take to arrive at its intended destination. The particular route used for a given connection is normally assigned during an initial call set up operation. Once a communication connection has been established, the route that the communication signals take through the system for the connection is generally fixed until an event occurs that requires a route change.
As the satellites in the satellite communication system rotate within their orbits, it often becomes necessary to change the particular satellite that is providing the link to a given subscriber. This will become necessary, for example, when the quality of the link between a current satellite and the subscriber has degraded below a threshold level. When this occurs, a satellite xe2x80x9chandoffxe2x80x9d operation will generally be performed to change the satellite providing the subscriber link from the current satellite (i.e., the first satellite) to a new satellite (i.e., the second satellite). As can be appreciated, a satellite handoff will change the route that communication packets will take through the system for the given connection.
In the past, a satellite handoff was performed by first requesting a communication channel within the second satellite to support the subscriber link. If a channel was available, the channel was reserved for the subscriber link and a handoff request signal was delivered to a remote entity at the other end of the communication connection (e.g., a gateway) requesting a handoff. Upon receiving the handoff request signal, the remote entity would determine a new route between the remote entity and the subscriber that included a direct link between the second satellite and the subscriber. All future packets delivered from the remote entity to the subscriber would indicate the new route in the associated routing information. The first satellite would then be notified of the route change and the direct link between the first satellite and the subscriber would be terminated.
While effective, the above-described handoff technique can take a relatively long time to accomplish. That is, the need to route a message back to the gateway before a handoff can be effected can create delays that can degrade call quality. In addition, situations might arise where the first satellite loses contact with the subscriber before the handoff to the second satellite is complete. This could result in, for example, a loss of data. Similar problems exist in other types of wireless communication systems such as, for example, terrestrial-based wireless networks.
Therefore, there is a need for a handoff technique for use in a wireless communication network that is capable of efficiently and rapidly handing off a communication link from a first node in the network to a second node in the network. The technique should be capable of use in a satellite communication system to handoff a subscriber link from a first satellite to a second satellite.